Generally, an inorganic constructional board used as wall backing material or the like are required to have various performance characteristics including nail-holding strength, bending strength and impact strength of more than definite values, fire resistance, water and humidity resistance, severability and the like. Therefore, a plaster board has been conventionally used for the inorganic construction board. However, the plaster board has a specific gravity of more than 0.74 and is required to have a certain degree of thickness in order to provide a desired bending strength. This will necessitate the use of a large-size plaster board, which involves excessive weight that is inconvenient from the standpoints of transport and work execution.
Further, Both surfaces of the plaster board are made of paper, which result in the following disadvantages. Once the plaster board has absorbed water, the strength is noticeably reduced. Moreover, when the paper of both surfaces of the plaster board has been separated, the strength, water resistance and the like are extremely deteriorated.
Therefore, such plaster board cannot be used in a highly humid underground location, kitchen having a source of vapor generation or the like. For this reason, instead of the plaster board, a calcium silicate board which has superior water and humidity resistance is used in such locations.
However, the calcium silicate board has a large specific gravity (0.9 or more), which involves great weight per piece and inconvenience from the standpoints of transport and work execution.
Although the calcium silicate board is used as sheathing material, for example, eaves ceiling by virtue of their excellent resistance to water and humidity, there is a problem that such board cannot be used in a very cold region because the freezing and thawing endurance of the board is of the order of about 50 cycles (which corresponds to two to three years of use).